Add repohasfile filter to Zoekt codepath

Created by: attfarhan

Addresses https://github.com/sourcegraph/sourcegraph/issues/4134.

Adds a new search token called repohasfile, which tells search to only include results from repositories that include certain files.

Note: this currently only works if there is another search term in addition to the repohasfile value, so the query repohasfile:\.md would return no results. Is this sufficient for the customer's needs @slimsag?

Test plan: unit tests were added.